In today's world, re-circulated exhaust gas (“EGR”) is utilized in internal combustion engines to assist in the reduction of throttling losses at low loads, to improve knock tolerance, and to reduce the level of oxides of nitrogen (“NOx”) in the exhaust gas. EGR is especially important as an emissions reducer in internal combustion engines that run lean of stoichiometry and are thus prone to emitting higher levels of NOx emissions.
Internal combustion engines that include exhaust gas re-circulation systems may rely upon internal EGR (IEGR), external EGR (EEGR), or a combination of the two. EEGR involves introduction of EGR into an engine combustion chamber through an intake valve after the EGR has traveled through an external conduit from the exhaust system. IEGR involves introduction of EGR into an engine combustion chamber through an exhaust valve or an intake valve without use of an external conduit. In order to provide exhaust flow to the combustion chambers when using EEGR, a pressure differential is needed between the exhaust flow path of the engine and the location in the intake system where the exhaust gas is reintroduced. For IEGR, an intake event (i.e., expansion of the volume within the combustion chamber, such as during the intake stroke of a piston in an internal combustion engine), typically provides a suitable pressure differential.
An IEGR system may take advantage of this pressure differential by opening one or more exhaust valves during the intake event of the valve's associated cylinder. A camshaft may be configured to facilitate selective activation and deactivation of valve-timing schemes, enabling IEGR to be selectively activated and/or deactivated. Duration, timing, and valve lift (i.e., flow rate) are affected by geometry of the camshaft in cooperation with the components of the valve train. Switchable rocker arms can facilitate switching between sets of lobes on a modified camshaft to enable switching between EGR modes. For example, a variable rocker arm assembly may be actuated or switched based on oil pressure, which can be modulated by an oil control valve. As different modes are actuated, different cam lobes become active, resulting in control over timing of valve actuation and thus control over IEGR.
During initial stages of engine operation following a cold start (i.e., approximately 200 seconds), before an engine reaches normal operating temperatures (e.g., coolant temperatures exceeding approximately 90 degrees C.), exhaust emissions may tend to exceed desirable or permissible levels. At relatively low exhaust temperatures, such as during engine warm-up, EEGR may negatively impact combustion stability and may also cause increased hydrocarbon (HC) emissions. IEGR may be useful in a strategy for DOC heating.
Fuel based warm-up strategies involving adjustments (retarding) to main injection timing and use of late post-injections have been used to accelerate the exhaust warm-up phase. Unfortunately, however, such practices can negatively impact HC emissions before the catalyst light-off and may also impact combustion stability with EEGR. IEGR provides transfer (i.e., recovery) of energy from a previous cycle to current cycle (recovering heat that would otherwise be discharged with exhaust gas and preheating the charge in the combustion chamber) while reducing heat losses with flow transport through the relatively high temperature exhaust system. As a result, IEGR facilitates increased temperatures of the air-fuel mixture prior to combustion. The resulting elevated combustion chamber temperatures can reduce ignition delays and associated emissions of CO and HC, particularly during cold engine operation. In addition, IEGR may result in increased exhaust gas temperatures, improving effectiveness of after-treatment components and thereby further reducing emissions of HC and CO.
Accordingly, it is desirable to have an improved method for controlling exhaust gas re-circulation in an internal combustion engine.